1. Field of the Invention
This invention relates to time delay networks for antenna beam steering, and more particularly to time delay networks that operate in the optical regime for steering a phased array antenna.
2. Description of the Related Art
The advent of electronically controlled phase shifters and switches for modern radar applications has focused attention on array antennas. With this type of antenna the aperture excitation can be modulated by controlling the phase of the individual radiating elements to produce beams that are electronically scanned. The principle of operation is that an antenna beam points in a direction normal to its phase front. With phased arrays, the phase front is adjusted to steer the beam by individual control of the phase of excitation for each radiating element. Phased array radar antennas are described in general in Radar Handbook 2d Ed., Ed. by M. Skolnik, Chap. 7, "Phased Array Radar Antennas", by T. Cheston and J. Frank, McGraw-Hill, 1990, pp. 7.1-7.82.
The general arrangement of a phased array radar system is illustrated in FIG. 1. Only four antenna radiating elements E1-E4 are shown for simplicity. An RF (radio frequency) generator 2 produces a microwave signal at the desired frequency to be radiated by the antenna. (While radar principles have been applied to signals ranging from a few megahertz to well beyond the optical region, covering a frequency extent on the order of 1,000,000,000:1, many systems operate in the microwave range of about 10.sup.9 -10.sup.11 Hz.) The RF signal is applied to four separate delay networks DY1-DY4 that are associated with respective radiating elements E1-E4. The various delay networks are programmed by an array computer 4 to delay the received RF signal by the appropriate amount for their respective radiating elements. The delayed signals are then provided to respective transmit/receive (TR) modules TR1-TR4 that are integrated with, or in very close proximity to, the radiating elements E1-E4. The TR modules provide the necessary broadcast power generation, and may be used in lieu of a central power source with a more lengthy distributed feed system to each of the radiating elements. Each TR module may control either one or a group of radiating elements.
The delay elements DY1-DY4 are programmed to produce progressively increasing delays from one end of the array to the other. In the illustration of FIG. 1, element E1 begins to radiate at time t.sub.0, element E2 at time t.sub.0 +.DELTA.t, E3 at t.sub.0 +2.DELTA.t and E4 at t.sub.0 +3.DELTA.t. The result is a transmitted wavefront 6 that is angled to the array, with the beam pointing in a direction 8 normal to the wavefront.
Numerous advanced radar designs for mobile, groundbased, airborne and space-borne applications call for compact, lightweight systems that have extremely wide instantaneous bandwidths, or that can operate at multiple frequency bands, to detect a wide range of object sizes and features. Wideband signals can cause beam "squint" (narrowing) and pattern degradation in phased array antennas that use conventional microwave phase shifter techniques. Squint and degradation are avoided by using beam forming networks with true time delay, which accomplish beam steering by using group delays rather than phase delays.
True time delay steering of phased array antennas has previously been accomplished with the use of coaxial cables connected in an electrical delay network. In such systems, time delay beam steering has generally been used at the sub-array level for coarse beam steering, with phase shifters used at the radiating element level to provide the fine beam steering. Coaxial cables, however, are relatively large and heavy, produce an undesirably high level of dispersion, and are subject to cross-talk. Consequently, because of their physical size, coaxial cables are not very useful for phased array antennas that require extremely long delay times.
A more compact and lightweight system for achieving time-delay beam steering consists of an optical delay network that uses fiber optic rather than coaxial delay lines. Because of the low microwave dispersion and crosstalk of the optical fibers, a delay network for multibands of microwave frequencies can be realized. This system has recently been disclosed in W. Ng et al., "Wide Band Fibre-Optic Delay Network for Phased Array Antenna Steering", Electronics Letters, Vol. 25, No. 21, Oct. 12, 1989, pp. 1456-57; and in W. Ng et al., "Optical Steering of Dual Band Microwave Phased Array Antenna Using Semiconductor Laser Switching", Electronics Letters, Jun. 7, 1990, Vol. 26, No. 12, pp. 791-92. The disclosed system is designed to steer a dual band (L and X band) phased array antenna, and is essentially "squintless". Eight fiber optic delay lines were connected in parallel, with each line illuminated by a respective laser diode connected to the line by means of a separate optical fiber or "pigtail". The lengths of the delay fibers increased from fiber to fiber in a linear progression, providing eight selectable delay periods for three bits of resolution. A desired time delay is selected by switching on the bias current for the laser diode that is pigtailed to the associated delay fiber. The antenna signal to be delayed is used to modulate the light emitted by the selected laser diode. The time delayed antenna signal is then decoded by a detector that is connected to the other end of the delay fibers.
Although the fiber optic system described above represents a significant improvement over conventional cable or waveguide techniques, it does have several shortcomings. One is an unbalance in the RF signal levels in each delay element that is caused by the variability of the pigtailing process used to connect the lasers to the various fibers. The other is the large number of delay elements needed for high-resolution systems. For n bits of resolution, the parallel-line approach described above requires 2n delay lines, so that an 11-bit system would need 2048 delay lines, an impractically large number. There is thus a need for a time delay network for steering a phased array antenna that avoids the limitations of the prior coaxial cable networks, and yet has better RF transfer uniformity and requires fewer delay lines than the prior fiber optic system.